The present invention relates to a jolt control system for a drive system including a prime mover and an automatic transmission.
An automatic transmission is shiftable between two gear positions by hydraulically activating friction elements selectively. The quality of a 1-2 upshift in the automatic transmission may be evaluated by inspecting a waveform of the transmission output torque. FIG. 10 shows in the fully drawn curve a a waveform of transmission torque which induces a substantial shift shock. The magnitude of the shift shock is determined on how much extra capacity of the shifting friction element exists in comparison with the output torque of the prime mover. If this capacity is excessively small, the friction element slips for a prolonged time, causing deterioration in its operating life and, in worst case, becomes baked. Hence, in the usual case, the capacity of the friction element must be set larger than required.
For reducing the shift shock, the capacity of the friction element is reduced by decreasing the servo activating hydraulic pressure supplied thereto during the shift operation. In this case, however, the time required for the shift operation becomes long as will be apparent from a waveform illustrated by the one-dot chain curve b in FIG. 10.
Alternatively, the output torque of the prime mover may be reduced. However, even with this measure, there still occurs a peak torque as illustrated by the dotted curve c in FIG. 10. The engine output torque control mentioned above, however, is not conducted when the engine has not been warmed up or operates at low speeds since were it conducted then the engine might stall. If the setting is made such that the level of servo activating hydraulic pressure supplied to the shifting friction element is sufficiently low to meet the reduced engine output torque, the capacity of the friction element becomes short when the engine output torque does not drop owing to the above-mentioned reasons. This causes the friction element to slip for a prolonged time, resulting in deterioration in the operating life of the friction element and, in worst case, baking. Besides, the quality of shift shock occurring when the engine output torque control is not conducted at low engine temperature or at low speed is different from that when the engine output torque control is conducted under those conditions. This gives a discomfort to the vehicle occupant.
Tokkai Sho 62-173337 (JP-A) teaches reducing the level of servo activating hydraulic fluid pressure supplied to the shifting friction element (in order to modulate the capacity of the shifting friction element) only when the engine output torque reduction control is to be conducted. In FIG. 10, the two-dot chain curve d shows a waveform of the engine output torque due to a jolt control system according to this teaching.
An object of the present invention is to provide a jolt control system wherein the capacity of the shifting friction element is varied to meet the amount of a reduction in the engine output torque owing to the engine output torque control conducted during a shifting operation in the automatic transmission.